It is known that the reception of acoustic telemetry signals travelling through the drilling fluid, often referred to as mud pulse telemetry, is substantially degraded if the drilling fluid inside the drillpipe contains substantial quantities of gas. Gas is often injected into the drilling fluid during underbalanced drilling (or low-head drilling in which the well is not underbalanced, but the bottom hole pressure is reduced by the addition of gas).
Although some of the difficulty in signal reception is an inevitable consequence of the attenuation of the acoustic signal in its passage up the mud column, it is also impeded by the acoustic conditions at the top of the mud column inside the surface system. This is especially true when the gas is injected into the drilling mud in the surface system, where the pressure pulses are to be detected. Because of signal attenuation and impeded acoustic conditions in the surface system, the telemetry signal can often be degraded to a point where conventional mud pulse telemetry is either impossible or impractical.
UK Patent Application GB 2 333 787 A discloses a system for mud pulse telemetry in underbalanced drilling wherein a fluid flow meter is used. The signal from the flow meter is converted into a pressure signal by a differential pressure sensor and is thereafter scaled and recorded as a pressure signal. Thus, instead of measuring the pressure, the system disclosed in GB 2 333 787 A measures the flow rate of the mud. Such systems are prone to degraded signal to noise ratios due to for example noise introduced by the mud pumps and gas introduction system.